The present invention relates generally to diffractive optical elements and more specifically the invention pertains to a time delay device and process that uses a holographic grating and optical beam coupling in a rotating photorefractive crystal for the purpose of displaying and processing of time-dependent analog signals.
The existing standard device for displaying time-dependent signals is the sound spectrograph, or sonograph. It is an electro-mechanical device widely used for speech analysis. It displays the signal intensity vs. time as greylevel plots on a recording paper, the maximum time delay is typically set to be a few seconds. A point by point comparison between the sonograph and the photorefractive pirouette display is irrelevant because they operate on totally different principles.
There are a few differences as far as performance is concerned. Apart from not being as fast as the photorefractive pirouette display, one of the disadvantages of the sonograph is the dynamic range of display. The intensity range of speech is 50-60 dB, whereas that of the sonograph is typically about 12 dB. There exist several methods to enhance the dynamic range of the sonograph. For example the "sectioner" device plots the signal intensity at a specific point of time in its full dynamic range. A contour plot method is also used. These are indirect compensation methods which have limited applicability. The sonograph also handles fewer signal channels compared to the photorefractive pirouette display. An optical device related to the storage and recall of a sequence of images is the optical disk memory. However, the disk memory is not a display because only one piece of information is retrieved at a time.
The task of providing a photorefractive display system for processing analog signals is alleviated to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,387,955 issued to Ludman et al; and PA1 U.S. Pat. No. 4,359,259 issued to Horner et al.
None of the cited patents disclose a time delay device that uses a holographic grating and optical beam coupling in a rotating photorefractive crystal for the purpose of displaying and processing of time-dependent analog signals. The patent to Ludman et al discloses a multiplexer/demultiplexer made up of a single component in the form of a holographically formed reflective grating. The Horner et al patent is of similar interest.